The present invention relates to a new and improved method for making diorganomonothiophosphinate compounds. More particularly, it relates to a process in aqueous media wherein a diorganophosphine is oxidized to form a diorganophosphine oxide, which is thereafter converted in the presence of sulfur and base to the corresponding diorganomonothiophosphinate compound.
Monothiophosphinate compounds are known to be useful as metal collectors for beneficiating certain mineral values from ores by froth flotation methods. The Soviet authors, P. M. Solozhenkin et al, in an article entitled, "Flotation Properties of Sulfur-Containing Phosphorus Derivatives." Dokl. Akad. Nauk Tadzh. SSR 13, No. 4, 26-30 (1970), for example, disclose that diethylmonothiophosphinic acid is a useful collector for galena, pyrite and antimonite. From the results of their study, they concluded that dithiophosphinate compounds performed better than monothiophosphinate compounds in flotations of these particular minerals.
More recently, it has been discovered, as is disclosed in commonly-assigned, copending application Ser. No. 675,489, filed Nov. 28, 1984, that diorganomonothiophosphinates provide exceptionally good metallurgical performance in selective flotation of base metal sulfide minerals, such as those of copper, nickel, molybdenum, cobalt and zinc, with selective rejection of pyrite, pyrrhotite and other gangue sulfide minerals, at reduced dosages over a broad range of pH values.
Prior art methods for making diorganomonothiophosphinate compounds are known. In one method, a corresponding diorganothiophosphoryl chloride is hydrolyzed to provide the corresponding diorganomonothiophosphinate, as summarized by the equation: ##STR2##
Another method for making diorganomonothiophosphinates is disclosed in the above-cited article by Solozhenkin et al, which comprises reacting phosphorus trichloride with primary alcohol to form dialkoxy substituted secondary phosphine oxide, followed by reaction with a Grignard Reagent, sulfur, and acidification to diorganomonothiophosphinic acid as summarized by the following reaction sequence: ##STR3##
Still another method is disclosed in the article Hoffman, H. and P. Schellenbeck, "Dorstelling und Eigenschafter einiger Phosphorverlunding mit tert.-Butylgruppen". Chem. Ber. 99 1134 (1966). In accordance with this method, an alkyl or aryl-substituted phosphorus dichloride is reacted with a Grignard Reagent to form a secondary phosphorus chloride which is thereafter hydrolyzed to form the secondary phosphine oxide, or it is disclosed that a secondary phosphine may be oxidized directly with hydrogen peroxide to form the secondary phosphine oxide. The secondary phosphine oxide, in accordance with the disclosed process, may be sulfurized by adding sulfur to a solution of the secondary phosphine oxide in benzene and heating to form the corresponding diorganomonothiophosphinic acid solution in benzene. This method is summarized as follows: ##STR4##
These prior art methods of making diorganomonothiophosphinate compounds have several shortcomings. The first two methods are not very economical for commercial production and therefore cannot be used to provide commercial quantities of diorganomonothiophosphinates for use as flotation reagents at a commercially acceptable price. Moreover, in the latter method of Hoffman et al, the need to use organic solvents, such as benzene, because of the instability of the diorganomonothiophosphinic acid products produced by the process in aqueous media, generally renders the method both economically and environmentally unattractive.
It is noteworthy that in an article published in J. Org. Chem 27, 2198 (1962), it is disclosed that a secondary phosphine sulfide cannot be oxidized to form a diorganomonothiophosphinate, i.e., ##STR5##
Accordingly, to overcome the disadvantages of the prior art methods, it is an object of the present invention to provide a new and improved method for making diorganomonothiophosphinate compounds in aqueous media in a commercially economical manner from readily available starting materials.